On the Earth, the global sulphur cycle is strongly influence by micro-organisms, with both assimilation and dissimilation pathways being important. Sulphate reducing bacteria (SRB - described by Postgate as "a bizarre group of microbes") reduce sulphate to sulphide with concomitant oxidation of an organic substrate. This dissimilatory process in anaerobic environments can be of profound economic significance. On the positive side, certain styles of major base metal ore deposit owe their existence to the activities of SRB in producing the sulphide which precipitates the metals (especially lead and zinc). On the negative side, the generation of hydrogen sulphide by SRB can result in the 'souring' of hydrocarbon accumulations in sedimentary basins like the North Sea.
The global geodynamic cycles of sulphur and carbon today are linked through redox reactions (the "free oxygen cycle"). Kinetic isotope effects associated with biological processes mean that variations in the ratios of naturally occurring stable isotopes (such as 34S/32S and 13C/12C) are especially powerful tracers of mass transfer within the cycles. The importance of this is that we can use stable isotope ratio variations in the geological record to infer the operation of kinetic isotope effects and, by implication, of biological processes in the past. Canfield has recently argued that this approach identifies two important milestones in the biogeochemical evolution of our planet: the first around 2.2 billion years ago and the second around eight hundred million years ago. This provides a context for discussion of sulphur in Martian meteorites.